I've heard that each Max-licence included 9999 render nodes of mental ray. That is, if you have a licence of Max you can install it on 10 000 computer, work on only one of them, but you can do network rendering via Backburner on the other 9999 of them with out any extra cost.

Is it the same with iray? So if we have 5 computers with Titan X cards in them at the office, can we render on all of them with only one licence of Max?

I don't think you can render one image on multiple machines in the same way you can do with mental ray, but for animation, to split up the frame range between the multiple cards. Each card needs to stack the entire scene in memory to work efficiently.